1/ @BoilingPb's point below is correct. A better design is a filter designed to attach onto one side of a six-sided non-flexible box that is perhaps 300-400 L or so in volume. Assuming it sits on the ground, that leaves 4 sides for filters. https://twitter.com/CorsIAQ/status/1291246072791068673

2/ In addition to the pleated design of good filters to increase surface area and reduce face velocity, lowering face velocity further by using 2 to 4 filters on sides/top of rigid box w/ design for good seal will help lower P drop, protect fan, & increase overall effectiveness.

3/ The use of multiple filters might seem like a higher cost, but they can be employed for a longer period (flow spread across more than one). I and others have tweeted about proper care in replacing filters (mask, goggles, gentle placement in tie-down garbage bag, etc.).